United States Special Counsel Robert Mueller's investigative team had interviewed veteran British spy Christopher Steele, who compiled the explosive Trump-Russia dossier, according to a report.

The dossier contains a series of memos detailing an alleged collusion between Russian efforts to aid Donald Trump's 2016 presidential campaign. Steele was hired by the Washington firm.

The information provided by Steele may help Mueller and his team assess if contacts between the Trump campaign and potential Russian operatives resulted in a collusion, CNN has reported.

Mueller is now working to determine whether any of the series of contacts between Trump campaign associates and suspected Russian operatives broke US law.

The FBI and the US intelligence community last year took the Steele dossier more seriously than the agencies have publicly acknowledged. James Clapper, former director of national intelligence, said in a January 2017 statement that the intelligence community had "not made any judgment that the information in this document is reliable."

Ever since the dossier came to light in January, President Trump and his allies have downplayed the dossier and insisted that it is a complete work of fiction. In a series of tweets earlier this year, President Trump said the memos were written by a "failed spy" who had relied on "totally made-up facts by sleazebag political operatives."

Former FBI Director James Comey had also expressed concerns to his counterparts that if the FBI alone presented the dossier allegations, then the president-elect would view the information as an attempt by the FBI to hold leverage over him.

Retired Lt. Gen. Keith Kellogg, the chief of staff for the National Security Council, was interviewed on Thursday. Kellogg was shown information and asked questions related to President Trump's former national security adviser Michael Flynn, the Hill reported.

President Donald Trump's former national security adviser, retired Gen. Michael Flynn, first declined to comply with a Senate subpoena in May, asserting his Fifth Amendment rights.

Flynn resigned in February after reports he misled senior members of the White House about his contacts with Russian officials. Mueller has asked for documents concerning Flynn and a Justice Department warning about Flynn's ties to a Russian diplomat.